1. Field of the Invention
The present invention relates to a monitor for sensing pointing, angle, and/or divergence of an optical beam, suitable for use as part of a lithographic apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Although a patterning device is often used to create a circuit pattern, it can also be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
Photolithography (also called microlithography) is used for manufacturing of semiconductor devices. Photolithography uses electromagnetic radiation, such as ultraviolet (UV), deep UV or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to implement photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system typically includes an illumination system, a reticle (also called a mask) containing a pattern, such as, for example, a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist-covered semiconductor wafer. The illumination system illuminates a region of the reticle with a preferably rectangular slot illumination field. The projection system projects an image of the illuminated region of the reticle circuit pattern onto the wafer.
Instead of a mask, in some lithographic apparatus, the patterning device can be a patterning array that comprises an array of individually controllable elements. Sometimes, the pattern can be changed more efficiently in a maskless system compared to a mask-based system. These types of apparatus are referred to as Optical Maskless Lithographic (OML) apparatus.
Known lithographic apparatus include so-called steppers or step-and-repeat apparatus, and so-called scanners or step-and-scan apparatus. In a stepper each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and the wafer is moved by a predetermined amount to a next position for a subsequent exposure. In a scanner, each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction, and next the wafer is moved to a next position for a subsequent exposure.
Some illumination systems include an array or diffractive scattering optical element positioned before, or optically ahead of, the reticle. The scattering optical element produces a desired angular light distribution that is subsequently imaged or relayed to the reticle.
Some illumination systems include a beam pointing sensor. Many beam pointing sensors typically measure the positional shift of a beam after traversing a transform lens. The transform lens serves to change beam pointing (position) variation into lateral beam shift (angle). The shift is then detected on an electronic position sensing device (PSD).
Some illumination systems include a beam divergence sensor. Many beam divergence sensors typically measure a change in beam size or footprint due to propagation of the beam over a known distance. However, most beam divergence sensor arrangements have limited sensitivity due to poor definition of illumination drop-off at an edge of the beam footprint. Additionally, ambiguity in measuring beam propagation distance reduces accuracy of the beam divergence determination.
Some illumination systems include a beam divergence sensor. Many beam divergence sensors typically measure a change in beam size or footprint due to propagation of the beam over a known distance. However, most beam divergence sensor arrangements have limited sensitivity due to poor definition of illumination drop-off at an edge of the beam footprint. Additionally, ambiguity in measuring beam propagation distance reduces accuracy of the beam divergence determination.
However, most sensor designs fail to determine beam pointing variation with a precision that is desired. For example, the sensitivity of known detector technology limits the detectable beam pointing variation to tenths of milli-radians of angular change. What is needed is an optical apparatus for monitoring and/or sensing a beam pointing, positioning variation, and/or divergence of a beam of light.